In this study, we analyze a resonant coupled wireless power transmission system with a negative impedance converter (NIC). The expressions for the output power of this system are obtained. The constraints of system parameter selection are determined according to the practical limitations of the NIC. The energy efficiency ratio (EER) is introduced to represent the relationship between the increase in system output power and the additional loss caused by the introduction of the NIC. The influence of the NIC on the EER was tested by changing the negative resistance. The experimental results show that the NIC reduces the loop impedance, increases the loop current, and improves the output power of the system. In addition, setting the appropriate parameters of the NIC can effectively increase the EER of resonant coupled radio power transmission by more than 4%.